A hydraulic pump control system for a construction machine according to the prior art as shown in FIG. 1 includes:
a variable displacement hydraulic pump (hereinafter, referred to as “hydraulic pump”) 1 and a pilot pump 2, which are connected to an engine;
a boom cylinder 3 and a swing motor 4, which are connected to and driven by the hydraulic pump 1;
control valves 5 and 6 that is installed in a center bypass path 1a of the hydraulic pump 1 and controls a start, a stop, and a direction change of the boom cylinder 3 and the swing motor 4 during shifting, respectively;
a remote control valve (RCV) 7 that supplies pilot signal pressures P3, P4, P1 and P2 to the control valves 5 and 6 to drive the boom cylinder 3 and the swing motor 4;
an orifice 8 that is installed on the lowermost stream side of the center bypass path 1a to generate a negative signal pressure; and
a regulator 9 that receives the signal pressure generated from the orifice 8 and controls a swivel angle of a swash plate of the hydraulic pump 1 to control a discharge flow rate of the hydraulic pump 1.
In the hydraulic pump control system for a construction machine as constructed above, the control valve 6 is shifted by the pilot signal pressures P1 and P2 supplied from the pilot pump 2 by manipulating the remote control valve 7, so that a hydraulic fluid discharged from the hydraulic pump 1 can be transferred to the swing motor 4 via the control valve 6 to drive the swing motor 4 to cause the upper swing structure to be swingably rotated with respect to the lower traveling structure.
In addition, the manipulation of the remote control valve 7 causes the control valve 5 to be shifted by the pilot signal pressures P3 and P4 supplied from the pilot pump 2, so that the hydraulic fluid discharged from the hydraulic pump 1 can be transferred to the boom cylinder 3 via the control valve 5 to drive the boom cylinder 3 to upward or downward move the boom.
In this case, the flow rate of the hydraulic fluid discharged from the hydraulic pump 1 is in proportional to a manipulation angle or a pilot signal pressure of the remote control valve 7 irrespective of a load pressure generated from a work apparatus such as a boom or the like. For this reason, if a lever of the remote control valve 7 is manipulated with a full stroke or a set pilot signal pressure exceeds a predetermined pressure, the hydraulic pump 1 discharges the hydraulic fluid at the maximum flow rate.
If an operator manipulates the remote control valve 7 abruptly to drive the swing motor 4, the flow rate of the hydraulic fluid discharged from the hydraulic pump 1 is increased in proportional to a manipulation amount of a swing manipulation lever to cause the upper swing structure to swingably rotated at a sudden acceleration with respect to the lower traveling structure. That is, the flow rate of the hydraulic fluid supplied to the swing motor 4 initially starts from “0” and is increased gradually until the swing motor 4 starts to be accelerated at its stop state and is increased in a swing speed.
Meanwhile, when the upper swing structure starts to be swingably rotated abruptly with respect to the lower traveling structure, all the hydraulic fluids corresponding to a difference between a hydraulic fluid of a flow rate sucked by rotation of the swing motor 4 and a hydraulic fluid of a flow rate discharged from the hydraulic pump 1 is returned to a hydraulic tank T through a relief valve 10 to protect the swing motor 4 and the swing structure. That is, if the hydraulic fluid of the flow rate discharged from the hydraulic pump 1 is increased to cause a discharge pressure of the hydraulic fluid to exceed a predetermined pressure of the relief valve 10, a hydraulic fluid of a flow rate other than a hydraulic fluid of the flow rate used to rotate the swing motor 4 is returned to the hydraulic tank T.
As shown in FIG. 2, a high flow rate of a hydraulic fluid is not required in an operation period from a time point when the upper swing structure starts to swing to a time point when the upper swing structure is accelerated. On the other hand, since the hydraulic pump 1 discharges the maximum flow rate of a hydraulic fluid, there occurs a problem in that a flow rate loss (i.e., a shaded region in FIG. 2) is caused, and the amount of fuel consumed by the engine is increased to decrease the fuel efficiency of the equipment.